Air-conditioning apparatus having a compressor-motor-connection switcher

ABSTRACT

An air-conditioning apparatus including a compressor incorporating an electric motor; a temperature sensor configured to detect indoor temperature; a drive circuit configured to drive the electric motor; a connection switching device configured to switch connection of stator windings of the electric motor between a first connection state and a second connection state higher in line-to-line voltage than the first connection state; and a controller configured to enter thermo-off when the indoor temperature reaches a target temperature or a correction temperature set based on the target temperature and cause the connection switching device to switch connection, the thermo-off being entered by stopping the compressor via the drive circuit. When a thermo-off count reaches a reference count with the electric motor being in the first connection state, the controller causes the connection switching device to switch connection from the first connection state to the second connection state.

CROSS REFERENCE TO RELATED APPLICATION

This application is a U.S. national stage application ofPCT/JP2017/029516 filed on Aug. 17, 2017, the contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an air-conditioning apparatus equippedwith a compressor, and more particularly, to connection switching ofstator windings of an electric motor incorporated in the compressor.

BACKGROUND ART

Generally, the largest proportion of electric power needed for operationof an air-conditioning apparatus is consumed by a compressor, andconsequently, energy efficiency of the air-conditioning apparatusdepends greatly on the compressor. In recent years, houses have becomeincreasingly airtight and thermally insulated, increasing occurrencefrequency of a low-load region, and in particular, operation efficiencyof compressors during low-speed operation of the compressors has beenincreasing in importance. On the other hand, considering situations suchas quick startup of cooling in extreme heat and quick startup of heatingat very low outside temperature, it is not that demand for high capacityachieved by increasing the rotational speed of the compressor to thelimit has declined. That is, recent air-conditioning apparatuses arerequired to satisfy two extremes of energy efficiency in a low-loadregion and high capacity in a high-load region, and technology aimed atcombining increased operation efficiency and an expanded range ofmovement of the compressor has been under development.

Incidentally, as an electric motor for a compressor, a permanent magnetmotor driven by an inverter is in common use, where the permanent magnetmotor uses permanent magnets for a rotor. The permanent magnet motor,which can operate on a small amount of current if the number of turns instator windings is increased, is capable of high-efficiency operationwith inverter losses due to current being reduced. On the other hand,there is a problem in that an induced voltage increases, causing a motorvoltage to reach a maximum output voltage of the inverter at arelatively low rotational speed and disabling the motor from operatingat a higher rotational speed. Conversely, with the permanent magnetmotor, if the number of turns in the stator windings is decreased, theinduced voltage decreases, allowing the motor to operate at up to ahigher rotational speed, but there is a problem in that a stator currentincreases, increasing inverter losses. In this way, permanent magnetmotors that have high efficiency at low-speed rotation cannot operate atup to high-speed rotation and permanent magnet motors that can operateat up to high-speed rotation have low efficiency at low-speed rotation.

To solve this problem, a system has been proposed that switches windingspecifications of an electric motor depending on whether the motor isoperating in a low-load region or high-load region. For example, PatentLiterature 1 proposes a system provided with a connection switchingdevice configured to switch a connection method for stator windings ofan electric motor between a star connection and delta connection inresponse to instructions. With this system, when an operating frequencyof the electric motor is lower than a predetermined frequency, the starconnection suitable for a low-speed range of a compressor is selected,and when the operating frequency is equal to or higher than thepredetermined frequency, the delta connection suitable for a high-speedrange of the compressor is selected. Also, Patent Literature 2 proposesa system that switches a coil connection in each phase between a serialconnection and parallel connection according to a deviation between aset temperature of an indoor unit and room temperature.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent No. 4722069

Patent Literature 2: Japanese Patent No. 5501132

SUMMARY OF INVENTION Technical Problem

However, in both Patent Literature 1 and Patent Literature 2, regardingconnection switching, it is necessary to perform a switching operationwith operation of the compressor stopped by stopping inverter outputfrom the viewpoint of product safety. In this case, the connectionswitching always involves a shutdown. Consequently, in practical use,even if energy efficiency in a low-load region and high capacity in ahigh-load region are fully satisfied, a function to maintain comfort,which is an essential function of the air-conditioning apparatus, mightbe impaired considerably.

The present invention has been made in view of the above problems andhas an object to provide an air-conditioning apparatus that combinesenergy efficiency in a low-load region and high capacity in a high-loadregion without impairing user comfort.

Solution to Problem

An air-conditioning apparatus according to an embodiment of the presentinvention comprises: a compressor incorporating an electric motor; atemperature sensor configured to detect indoor temperature; a drivecircuit configured to drive the electric motor; a connection switchingdevice configured to switch connection of stator windings of theelectric motor between a first connection state and a second connectionstate higher in line-to-line voltage than the first connection state;and a controller configured to enter thermo-off when the indoortemperature reaches a target temperature or a correction temperature setbased on the target temperature and cause the connection switchingdevice to switch connection, the thermo-off being entered by stoppingthe compressor via the drive circuit. Then, when a thermo-off countreaches a reference count with the electric motor being in the firstconnection state, the controller causes the connection switching deviceto switch the connection of the stator windings from the firstconnection state to the second connection state.

Advantageous Effects of Invention

The air-conditioning apparatus according to an embodiment of the presentinvention switches the electric motor from the first connection state tothe second connection state higher in line-to-line voltage than thefirst connection state in synchronization with thermo-off. This allowsconnection to be switched without increasing stop frequency of thecompressor from conventional stop frequency and thereby combines energyefficiency in a low-load region and high capacity in a high-load regionwithout impairing user comfort.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a configuration example of arefrigerant circuit of an air-conditioning apparatus according toEmbodiment 1 of the present invention.

FIG. 2 is a control block diagram of a control system of theair-conditioning apparatus including a control board of FIG. 1.

FIG. 3 is a timing chart of defrosting control.

FIG. 4 is a circuit diagram showing an example of a connection switchingdevice of FIG. 2, for stator windings of a compressor.

FIG. 5 is an external view of a compressor of FIG. 1.

FIG. 6 is a schematic diagram showing efficiencies of a star connectionand delta connection vs. compressor frequency as well as an intersectionof efficiency curves.

FIG. 7 is a schematic diagram showing an example of changes, with time,of compressor frequency in a low-load region during heating.

FIG. 8 is a flowchart (Part 1) of connection switching control forswitching stator windings of the electric motor from a delta connectionto a star connection, showing an example in which a thermo-off count andcompressor frequency are used as conditions.

FIG. 9 is a flowchart (Part 2) of connection switching control forswitching the stator windings of the electric motor from the deltaconnection to the star connection, showing an example in whichthermo-off time is reflected as a switching condition.

FIG. 10 is a flowchart (Part 3) of connection switching control forswitching the stator windings of the electric motor from the deltaconnection to the star connection, showing an example in which acompressor stop is reflected as a switching condition.

FIG. 11 is a schematic diagram showing an example of changes, with time,of compressor frequency during heating if load fluctuations occur.

FIG. 12 is a schematic diagram showing an example of changes, with time,of compressor frequency during heating if target value fluctuationsoccur.

FIG. 13 is a flowchart (Part 1) of connection switching control forswitching the stator windings of the electric motor from the starconnection to the delta connection, showing an example in whichoperating time and outdoor-side heat exchanger temperature are used asstart conditions of defrosting operation.

FIG. 14 is a flowchart (Part 2) of connection switching control forswitching the stator windings of the electric motor from the starconnection to the delta connection, showing an example in whichcompressor frequency is used as a start condition of defrostingoperation.

FIG. 15 is a flowchart (Part 3) of connection switching control forswitching the stator windings of the electric motor from the starconnection to the delta connection, showing an example in whichcompressor temperature is used as a start condition of defrostingoperation.

FIG. 16 is a schematic diagram showing another example of changes, withtime, of compressor frequency during heating if load fluctuations occur.

FIG. 17 is a variation of FIG. 14 showing an example in which acompressor operating current is used as a start condition of defrostingoperation.

FIG. 18 is a characteristics curve showing a relationship betweencompressor frequency and line-to-line voltage in the case of a starconnection and in the case of a delta connection.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described below withreference to the drawings. Description will be given below by taking asan example a case in which stator windings are connected by a deltaconnection in a first connection state while the stator windings areconnected by a star connection in a second connection state. However,the types of first connection state and second connection state do notmatter as long as a magnitude relationship between line-to-line voltagesdoes not change. For example, even if a serial connection and parallelconnection are combined as with Patent Literature 2, i.e., each phase ofthe stator windings is made up of plural windings, and in the firstconnection state, the stator windings are connected in parallel on aphase by phase basis while in the second connection state, the statorwindings are connected in series on a phase by phase basis, similareffects are obtained.

Embodiment 1

FIG. 1 is a schematic diagram showing a configuration example of arefrigerant circuit of an air-conditioning apparatus according toEmbodiment 1 of the present invention. The air-conditioning apparatusincludes an indoor unit 1 placed on an indoor side to be air-conditionedand an outdoor unit 2 placed on an outdoor side. The indoor unit 1includes an indoor-side heat exchanger 5. The outdoor unit 2 includes acompressor 3, a four-way valve 4, an outdoor-side heat exchanger 6, andan expansion valve 7. The compressor 3, four-way valve 4, outdoor-sideheat exchanger 6, expansion valve 7, and indoor-side heat exchanger 5are connected in an annular manner through refrigerant pipes, forming arefrigerant circuit 100. The four-way valve 4 is used to switch theair-conditioning apparatus between cooling operation and heatingoperation and is illustrated in FIG. 1 as having been switched tocooling operation. The refrigerant circuit 100 is filled withrefrigerant. The type of refrigerant is not specifically limited. Theindoor unit 1 further includes an indoor-side fan 8 for use to send airto the indoor-side heat exchanger 5. The indoor-side fan 8 is placed onthe windward side of the indoor-side heat exchanger 5. Note that theindoor-side fan 8 may be placed on the leeward side of the indoor-sideheat exchanger 5. The outdoor unit 2 further includes an outdoor-sidefan 9 for use to send air to the outdoor-side heat exchanger 6. Theoutdoor-side fan 9 is placed on the leeward side of the outdoor-sideheat exchanger 6. Note that the four-way valve 4 may be replaced byanother selector valve having a similar function.

A temperature sensor 10 is attached to an outer shell of the compressor3 of the outdoor unit 2. The temperature sensor 10 is used to detecttemperature of the compressor 3. Note that the temperature sensor 10 maybe installed in another location as long as the temperature of thecompressor 3 can be estimated, and may be installed on a refrigerantpipe on a route from the compressor 3 to the four-way valve 4 instead ofthe outer shell of the compressor 3. A temperature sensor 11 is attachedto the indoor unit 1 on the windward side of the indoor-side fan 8. Thetemperature sensor 11 is used to detect air temperature before inflowinto the indoor-side heat exchanger 5, i.e., to detect room temperature.Note that the location of the temperature sensor 11 is not limited tothe one shown in FIG. 1 as long as the room temperature can be detected.A temperature sensor 12 is attached to a pipe wall of a refrigerant pipeserving as a refrigerant inlet of the outdoor-side heat exchanger 6during heating. The temperature sensor 12 is used to detect evaporatingtemperature when the outdoor-side heat exchanger 6 functions as anevaporator. According to the present embodiment, because the temperaturedetected by the temperature sensor 12 is used for defrosting operation,the temperature sensor 12 may be installed in another location as longas the temperature of the outdoor-side heat exchanger 6 can beestimated. Also, the number of temperature sensors is not limited to 3shown in FIG. 1, and may be more than 3. Information on the temperaturesdetected by the temperature sensors 10 to 12 is outputted to a controlboard 20 provided in the outdoor unit 2. Details of the control board 20will be described later.

Next, operation of the air-conditioning device of FIG. 1 will beoutlined. As described above, refrigerant is enclosed in the refrigerantcircuit 100 and is compressed by the compressor 3. During cooling,refrigerant compressed by the compressor 3 is condensed and liquefied bythe outdoor-side heat exchanger 6, expanded by the expansion valve 7,and then evaporated by the indoor-side heat exchanger 5, subsequentlyreturning to the compressor 3 and thereby going through a refrigerationcycle formed by a cooling circuit. During heating, refrigerantcompressed by the compressor 3 is condensed and liquefied by theindoor-side heat exchanger 5, expanded by the expansion valve 7, andevaporated by the outdoor-side heat exchanger 6, subsequently returningto the compressor 3 and thereby going through a refrigeration cycleformed by a heating circuit.

When performing cooling or heating as described above, theair-conditioning device of FIG. 1 controls various parts such that thetemperature detected by the indoor-side temperature sensor 11 willconform to a target value. That is, the air-conditioning device controlsrotational speed of the compressor 3, an opening degree of the expansionvalve 7, a volume of air sent by the indoor-side fan 8, and a volume ofair sent by the outdoor-side fan 9. The control is performed based onthe temperatures detected by the temperature sensor 10, temperaturesensor 11, and temperature sensor 12 and cooling capacity or heatingcapacity is controlled. This control is performed by the control board20 of the outdoor unit 2.

FIG. 2 is a control block diagram of a control system of theair-conditioning apparatus including the control board 20 of FIG. 1. Acontroller 21, storage device 22, drive circuit 23, and connectionswitching device 24 are mounted on the control board 20. The controller21 is made up of a microprocessor or DSP (digital analog processor). Thecontroller 21 incorporates a timer 21 a. The storage device 22 prestoresa reference count of thermo-off and stores a thermo-off count, the timesat which thermo-off occurred, and other similar data. The controller 21is connected with the drive circuit 23. The controller 21 controlsoutput frequency of a three-phase AC voltage outputted from the drivecircuit 23. The drive circuit 23 outputs the three-phase AC voltageusing an inverter and drives a three-phase permanent magnet motor 25incorporated in the compressor 3. On instructions from the controller21, the connection switching device 24 switches a connection method forthe stator windings of the permanent magnet motor 25 from the deltaconnection to the star connection or from the star connection to thedelta connection. Details of the connection switching device 24 will bedescribed later. Note that the electric motor incorporated in thecompressor 3 may be a three-phase motor other than a permanent magnetmotor.

The air-conditioning apparatus of FIGS. 1 and 2 performs thermo-offcontrol and defrosting operation such as described below. First, thethermo-off control and defrosting operation will be outlined.

(Thermo-Off Control)

In the case of heating operation, when the room temperature detected bythe temperature sensor 11 exceeds a target temperature and reaches athermo-off temperature, the controller 21 stops the compressor 3.Subsequently, when the room temperature falls below the targettemperature and reaches a thermo-on temperature, the controller 21resumes operation of the compressor 3. In the case of cooling operation,procedures are similar to the heating operation. When the roomtemperature detected by the temperature sensor 11 falls below a targettemperature and reaches a thermo-off temperature, the controller 21stops the compressor 3. Subsequently, when the room temperature exceedsa thermo-on temperature, the controller 21 resumes operation of thecompressor 3. Note that thermo-off temperature is the targettemperature+α in the case of heating, and the target temperature−α inthe case of cooling. The thermo-on temperature is the targettemperature−β in the case of heating, and the target temperature+β inthe case of cooling. Here, α and β are values set as appropriate toavoid hunting resulting from on-off action. The thermo-off temperaturecorresponds to a correction temperature of the present invention, andequals the target temperature when a is 0.

(Defrosting Operation)

FIG. 3 is a timing chart of defrosting control. The defrosting operationis an example of operation performed conventionally, and will beoutlined below. After defrosting operation is finished, when a definiteperiod of time elapses after resumption of heating operation (conditiona) and the temperature detected by the temperature sensor 12 falls to orbelow a prescribed value Tdef_on (condition b), the controller 21determines that conditions for starting defrosting operation aresatisfied and starts defrosting operation. That is, the controller 21determines that frost has formed on the outdoor-side heat exchanger 6and starts defrosting operation.

In defrosting operation, after stopping the compressor 3, the controller21 forms a cooling circuit by switching the four-way valve 4 and meltsfrost by a reverse defrosting method that involves restarting thecompressor 3 and circulating the refrigerant. The defrosting operationis continued until the temperature detected by the temperature sensor 12reaches or exceeds a prescribed value Tdef_off. When the temperaturedetected by the temperature sensor 12 reaches or exceeds the prescribedvalue Tdef_off (condition c), the controller 21 determines that acondition for finishing the defrosting operation is satisfied andfinishes the defrosting operation. In finishing the defrostingoperation, the controller 21 stops the compressor 3 first, returns tothe heating circuit by switching the four-way valve 4, and resumesheating operation by restarting the compressor 3. Note that according tothe present embodiment, when the stator windings of the permanent magnetmotor 25 are star-connected, defrosting operation is also performedunder other conditions in addition to the above conditions.

FIG. 4 is a circuit diagram showing details of the connection switchingdevice 24. FIG. 5 is an external view of the compressor 3. As describedabove, the compressor 3 incorporates the permanent magnet motor 25. Thestator windings of the permanent magnet motor 25 are connected to thedrive circuit 23 and connection switching device 24. Three C contactrelays 24 a, 24 b, and 24 c are used as the connection switching device24. The permanent magnet motor 25 has six leads. The six leads areconnected to the outside of the compressor 3 through glass terminals 16a and 16 b shown in FIG. 5. Three of the six leads are connected to thedrive circuit 23, and the remaining three leads are connected to thethree C contact relays 24 a, 24 b, and 24 c, respectively.

An a-contact of the C contact relay 24 a is connected to a W phaseoutput terminal of the drive circuit 23, and an a-contact of the Ccontact relay 24 b is connected to a V phase output terminal of thedrive circuit 23. An a-contact of the C contact relay 24 c is connectedto a U phase output terminal of the drive circuit 23. A b-contact of theC contact relay 24 a, a b-contact of the C contact relay 24 b, and ab-contact of the C contact relay 24 c are connected with one another. Inthe connection switching device 24 configured in this way, the deltaconnection is formed when the relay contacts of the three C contactrelays 24 a, 24 b, and 24 c are switched to the a-contact side, and thestar connection is formed when the relay contacts are switched to theb-contact side.

Incidentally, power is consumed when relay coils are energized.Generally, integral power consumption of the relays is reduced on anannual basis if relay coils are not energized in a low-speed range ofthe compressor, which occurs frequently. Thus, according to the presentembodiment, the connection switching device 24 is configured such thatthe star connection will be used when the coils are not energized andthat the delta connection will be used when the coils are energized.However, if frequency at which loads are generated differs from ageneral tendency, the connection switching device 24 may be configuredsuch that the star connection will be used when the coils are energizedand that the delta connection will be used when the coils are notenergized.

The controller 21 performs capacity control in relation to an operatingstate of the refrigeration cycle based on the temperatures detected bythe plural temperature sensors 10 to 12. In the course of the control ifconditions for switching to the star connection are satisfied duringoperation with the delta connection the controller 21 switches theconnection from the delta connection to the star connection bycontrolling the connection switching device 24. In this case, the relaycontacts of the C contact relays 24 a, 24 b, and 24 c of the connectionswitching device 24 are switched from the a-contacts to the b-contacts.On the other hand, if conditions for switching to the delta connectionare satisfied during operation with the star connection, the controller21 switches the connection from the star connection to the deltaconnection by controlling the connection switching device 24. In thiscase, the C contact relays 24 a, 24 b, and 24 c of the connectionswitching device 24 are switched from the b-contacts to the a-contacts.Furthermore, at the same time with relay switching, the controller 21switches various control constants used to drive the compressor 3. Notethat relay switching is done when the compressor 3 is stopped by takingindividual variation of the three relays and product safety intoconsideration.

Generally, when a line-to-line voltage is high, because an amount ofcurrent required to generate necessary torque is reduced, a motor hashigh efficiency, but undergoes an increase in an induced voltage.Furthermore, because the induced voltage is proportional to rotationspeed, the line-to-line voltage reaches a maximum inverter output at alower rotation speed. Conversely, when the line-to-line voltage is low,the motor can operate up to a higher rotation speed, but the currentneeded to generate necessary torque increases. For example, as shown inFIG. 18, it is a known fact that the line-to-line voltage in the starconnection is higher than the line-to-line voltage in the deltaconnection, and is √3 times the delta connection. If the star connectionthat develops a high line-to-line voltage is used in a low-speed range,which occurs frequently, and switching is done to the delta connectionwith the low line-to-line voltage when high-speed rotation becomesnecessary, demand for high capacity can be met while minimizing integralpower consumption on an annual basis.

However, if a threshold is determined based solely on compressorfrequency or inverter output voltage and switching is done between thedelta connection and star connection each time the threshold is crossed,a shutdown has to be caused each time the determined threshold iscrossed. Such a situation goes against the user's intent. Also, ifoperating time is used as a trigger, the compressor has to be stopped toswitch the connection, which increases the number of compressor stops ina season as a whole compared to conventional cases, resulting in lowercomfort than conventionally the case. Thus, it is desirable inmaintaining comfort to synchronize connection switch timings withconventional compressor stop timings and thereby not to generatecompressor stop timings more than conventionally the case.

Next, a technique for determining which of the star connection and deltaconnection is more effective in reducing power consumption will bedescribed using FIG. 6. FIG. 6 is a schematic diagram showingefficiencies of a star connection and delta connection vs. compressorfrequency as well as an intersection of efficiency curves. Compressorfrequency can be used as one of criteria for determining whetheroperation with the star connection is more efficient than operation withthe delta connection. The efficiencies of the star connection and deltaconnection form curves each having an efficiency peak and superiority inefficiency switches at an intersection (Nc) of the curves. Strictlyspeaking, the position of the intersection changes with environmentalfactors such as inside and outside temperatures, but the changes arenegligible in considering reductions in annual power consumption understandard conditions, and do not affect the general trend. Regarding afrequency threshold for switching control between the star connectionand delta connection, it is sufficient that one efficiency intersectionunder standard conditions is established as a representative point. Whenit is expected that operation will be continued at frequencies in excessof the frequency threshold, the use of the delta connection iscontinued. Conversely, when it is expected that operation below thefrequency threshold will be predominant, the star connection is used asa principle.

Here, if a switching operation is performed depending only on whetherthe current compressor frequency is higher or lower than the operatingfrequency threshold as described above, the following problems arise.Erroneous determinations increase before operation stabilization such asright after the start of operation or at the time of sudden airconditioning load fluctuations, causing operation to be performed usinga wrong connection. Also, so-called hunting in which the star connectionand delta connection switch between each other repeatedly might occur,considerably impairing energy efficiency and comfort conversely. Aconnection switching operation provides higher efficiency and comfort asa whole if performed by waiting for the air-conditioning apparatus toenter stable operation.

In the present embodiment, connection is switched from the aboveviewpoint. Next, connection switching from the delta connection to thestar connection and connection switching from the star connection to thedelta connection will be described individually.

(Connection Switching from Delta Connection to Star Connection)

Conditions for switching from the delta connection to the starconnection will be described below using FIG. 7. FIG. 7 is a schematicdiagram showing an example of changes, with time, of compressorfrequency in a low-load region during heating. As the room temperaturedetected by the temperature sensor 11 approaches a target temperatureset by a user or a target temperature automatically set by theair-conditioning apparatus, the compressor frequency falls. When theroom temperature exceeds the target temperature and approaches thethermo-off temperature sufficiently, to inhibit overshoot or undershootin the room temperature and cut operating power at the same time,thermo-off occurs to stop the compressor 3 (A1, A2). Switching from thedelta connection to the star connection during the thermo-off eliminatesthe need to add a compressor stop for connection switching.

At the start of operation, operation is started using the deltaconnection characterized by a high-speed range of the compressor, highefficiency, and a wide range of movement. Then, subsequent operatingloads are estimated from the operating state, and when it is determinedby the apparatus that the use of the star connection for subsequentoperation will be more effective in cutting power consumption,connection is switched in synchronization with thermo-off. Thethermo-off count is stored in the storage device 22 of the control board20. Timing for switching from the delta connection to the starconnection comes when it is determined that operation with the starconnection will be effective in cutting power consumption. Specifically,this is when the number of compressor stops due to thermo-off reaches Ntimes and the frequency just before the thermo-off is equal to or lowerthan the frequency threshold all the N times. During the Nth thermo-off,an operation of switching to the star connection is performed. In FIG.7, N=2 as an example, but N may be an integer equal to or larger than 2and may be set as appropriate according to housing performance.

Also, for example, if a long time passes from the first count to the Nthcount there is no point in counting thermo-off cumulatively because itis likely that environmental conditions including outside temperaturehave changed. Thus, the condition that switching from the deltaconnection to the star connection is done if thermo-off occurs N timessuccessively within a definite period of time is more appropriate. Also,if a shift to defrosting operation takes place before the thermo-offcount reaches N, the thermo-off count is reset. That is, becauseair-conditioning conditions under which a shift to defrosting operationtakes place often require high capacity, the delta connection ismaintained.

Also, if thermo-off time continues for a long time, switching to thestar connection may be done by determining that the load is low, i.e.,high capacity operation is unnecessary even if the number of times hasnot reached N. In the operation with the star connection, a permanentmagnet of the electric motor is demagnetized at high temperatures moreeasily than in the operation with the delta connection. If refrigerantleaks, operation is performed on low refrigerant and dischargetemperature tends to rise. When the temperature detected by thetemperature sensor 10, i.e., the temperature of the compressor 3, isequal to or higher than a reference temperature, even if the conditionsfor switching to the star connection are satisfied, switching to thestar connection is not done from the viewpoint of compressor protection.

The controller 21 performs connection switching control by reflectingthe above-mentioned switching conditions and a process of the controlwill be described based on flowcharts. FIGS. 8 to 10 are flowchartsshowing a control process performed by the controller when switchingfrom the delta connection to the star connection, and computations forthe control process are performed concurrently.

FIG. 8 is a flowchart (Part 1) of connection switching control forswitching stator windings of the electric motor 25 from the deltaconnection to the star connection, showing an example in which thethermo-off count and compressor frequency are used as conditions. Thecontroller 21 determines whether indoor temperature has reached thethermo-off temperature (S11) and enters thermo-off when the thermo-offtemperature is reached (S12). With the compressor 3 stopped due to thethermo-off, the following processes are performed. The controller 21determines whether the compressor frequency is equal to or lower thanthe frequency threshold at the time when the indoor temperature reachesthe thermo-off temperature (or just before the thermo-off) (S13). Whenthe compressor frequency is equal to or lower than the frequencythreshold, the controller 21 increments the count n such that n=n+1(S14). The count n is stored in the storage device 22 and the value ofthe count n is updated. In so doing, the value of the count n and thetime at which the count n is reached are stored in the storage device 22as well. The count n indicates the thermo-off count when the indoortemperature reaches the thermo-off temperature, resulting in thermo-offand the compressor frequency just before the thermo-off becomes equal toor lower than the frequency threshold. Next, the controller 21determines whether the count n has reached a set value N (S15).

When it is determined that the count n has reached a set value N, nextthe controller 21 determines whether a definite period of time haspassed at present since the start of operation of the air-conditioningapparatus (S16). Note that the definite period of time in step 16 (S16)corresponds to a third reference time of the present invention and isset from the viewpoint that a shift to connection switching controlshould take place after the air-conditioning apparatus enters stableoperation. When it is determined that the definite period of time haspassed at present since the start of operation, next the controller 21determines whether the time required for the count n to increase from 1to N was within a reference time (S17). The reference time in step 17(S17) corresponds to a second reference time of the present inventionand is set from the viewpoint of determining whether the load is low.

When it is determined that the time required for the count n to increasefrom 1 to N was within the definite period of time, next the controller21 determines whether the temperature of the compressor 3 is lower thanthe reference temperature (S18). When it is determined that thetemperature of the compressor 3 is equal to or lower than the referencetemperature, the controller 21 switches from the delta connection to thestar connection by controlling the connection switching device 24 (S19).Here, the C contact relays 24 a, 24 b, and 24 c are switched from theb-contacts to the a-contacts. When the condition is not satisfied (NO)in the determination of any of steps 11, 15, 16 (S11, S15, S16), theprocessing is finished. Also, when the condition is not satisfied (NO)in any of steps 13, 17, 18 (S13, S17, S18), the controller 21 sets thecount n to n=0 (S20) and finishes the process of S20. Note that when thedetermination in step 18 (S18) is NO, i.e., when the temperature of thecompressor 3 is equal to or higher than the reference temperature, theprocess of S20 may be finished without setting the count n to n=0.

FIG. 9 is a flowchart (Part 2) of connection switching control forswitching the stator windings of the electric motor from the deltaconnection to the star connection, showing an example in whichthermo-off time is reflected as a switching condition. The process ofthe flowchart is started with a start of thermo-off serving as atrigger. When thermo-off is started (S12 in FIG. 8), the controller 21measures the time duration of the thermo-off state (S21). The timemeasurement is continued until thermo-on (S23), and when it isdetermined that the measured time exceeds the reference time (S22), nextit is determined whether a connection switching process has already beenperformed (S24). When it is determined that a connection switchingprocess has not been performed yet, the controller 21 goes to step 19(S19) of FIG. 8 and performs a connection switching process. Note thatthe reference time in step 22 (S22) corresponds to a first referencetime of the present invention.

FIG. 10 is a flowchart (Part 3) of connection switching control forswitching the stator windings of the electric motor from the deltaconnection to the star connection, showing an example in which acompressor stop is reflected as a switching condition. The process ofthe flowchart is started with a compressor stop serving as a triggerwhen the compressor stops due to defrosting or an abnormal condition.The controller 21 sets the count n to n=0, updates storage content ofthe storage device 22 (S26), and finishes the processing. Consequently,the count n in the flowchart of FIG. 8 is n=0. In this way, the count nis reset if the compressor stops due to defrosting or another factorbetween thermo-off and next thermo-off and connection is switched ifthermo-off occurs N times continuously in the flowchart of FIG. 8. Notethat when it is said that thermo-off occurs N times continuously, thismeans that the compressor does not stop due to another factor betweenthermo-off and next thermo-off.

The connection switching control described above is applied to either ofheating operation and cooling operation. Step (S13) of FIG. 8 may beomitted and the switching process may be performed based solely on thethermo-off count. Also, in the above description, the fact thatoperation of the air-conditioning apparatus is being continued in alow-load region is determined based on continuous occurrence ofthermo-off and operation in non-low-load region (i.e., high-load region)is determined based on occurrence of defrosting. However, if theair-conditioning apparatus has a means of detecting or estimating airconditioning loads, determination of air conditioning loads may be madebased on detection information or estimation information on the airconditioning loads.

As described above, the present embodiment switches connection from thedelta connection to the star connection, offering the followingadvantageous effects.

In the present embodiment, regarding the connection switching from thedelta connection to the star connection, when the thermo-off countreaches a reference count N, the connection switching device 24 switchesconnection from the delta connection to the star connection during athermo-off period. That is, switching from the delta connection to thestar connection is done in synchronization with thermo-off. This makesit possible to switch connection without increasing stop frequency ofthe compressor 3 compared to conventional stop frequency and therebyreduce unpleasantness caused by a compressor stop during connectionswitching. Besides, the shift from the delta connection to the starconnection improves energy efficiency. Also, accuracy of determinationas to which connection is more appropriate for operation is increased.This makes it possible to reduce unnecessary connection switching andmaintain comfort. In this way, the present embodiment combines energyefficiency in a low-load region and high capacity in a high-load regionwithout impairing comfort.

Also, in the present embodiment, connection is switched when thethermo-off count reaches the reference count N and a state in which anoperating frequency of the compressor 3 during thermo-off is equal to orlower than the frequency threshold continues to amount to the referencecount N. In this way, connection is switched by taking intoconsideration the thermo-off count and the operating frequency of thecompressor 3 during thermo-off. This further increases accuracy ofdetermination on switching to the star connection, making it possible tocombine energy efficiency in a low-load region and high capacity in ahigh-load region without impairing comfort.

Also, in the present embodiment, if thermo-off time reaches or exceedsthe first reference time, connection is switched regardless of thethermo-off count and compressor frequency. When a duration of thermo-offreaches or exceeds a definite period of time, high capacity operation ofthe compressor 3 is unnecessary and connection is switched from theviewpoint of energy efficiency.

Also, in the present embodiment, if the second reference time isexceeded during a period from the first count of thermo-off until thereference count N is reached, the thermo-off count is reset. If a longperiod of time is required until the reference count N is reached, thecompressor 3 is in a driving state that cannot be said to be in alow-load region. Therefore, the driving state with the stator windingsdelta-connected is maintained.

Also, in the present embodiment, when defrosting operation is performedto remove frost from the outdoor-side heat exchanger 6, the thermo-offcount is reset. The fact that defrosting operation was performed meansthat the compressor 3 is not in a low-load state, and thus operation iscontinued with the stator windings delta-connected.

Also, in the present embodiment, for the third reference time after thestart of operation of the air-conditioning apparatus until operation isstabilized, connection switching is not controlled regardless of thepresence or absence of the switching conditions. This is because for adefinite period of time after the start of operation of theair-conditioning apparatus, the operation is transitional and accuracyof load determination is low, and thus connection is switched when theoperation is stabilized.

Also, according to the present embodiment, when the temperature of thecompressor 3 is equal to or higher than a reference temperature,switching to the star connection is not done and operation is continuedusing the delta connection.

Also, according to the present embodiment, connection is switched in thethermo-off period in which the reference count is reached.

(Switching from Star Connection to Delta Connection)

Next, conditions for switching from the star connection to the deltaconnection will be described using FIGS. 11 and 12. FIG. 11 is aschematic diagram showing an example of changes, with time, ofcompressor frequency during heating if load fluctuations occur. Afteroperation is started using the delta connection, since a shift to thestar connection takes place upon entering stable operation in a low-loadregion as described above, the compressor 3 operates in the low-loadregion as a rule when the star connection is selected. Therefore, ashift from the star connection to the delta connection takes place whenthere is a sudden increase in the compressor frequency required as aresult of sudden room temperature fluctuations, a major change in theset temperature, or other conditions.

In such a case, even if the delta connection is superior in efficiency,if a shift to a connection switching operation is made by suddenlystopping operation in a state in which high capacity will be required,the user will feel odd. In that case, the room temperature is increasedby operating within a range of up to maximum capacity with the starconnection for a definite period of time, and then a shift is made to anoperation of switching to the delta connection. However, even with thestar connection, because the operation during the definite period oftime involves high capacity operation, evaporating temperature ofrefrigerant in the outdoor-side heat exchanger 6 falls, producing froston the outdoor-side heat exchanger 6. When only connection switching isdone with frost still remaining and the compressor 3 is restarted withthe delta connection, time wasted on the compressor stop is added andfrost further grows after the switching to the delta connection,subsequently accelerating the timing of shifting to defrostingoperation. Thus, during switching operation, even if the value detectedby the temperature sensor 12 is above the prescribed value, thedefrosting operation supposed to be performed subsequently is performedearlier.

At time B1 in FIG. 11, for example, a window or door is fully opened,causing a sudden air conditioning load fluctuations and the roomtemperature falls abruptly. In this case, because the compressorfrequency exceeds a frequency threshold Nc, the operation runs in aregion in which the delta connection should be selected, and if oneattempts to switch the connection just when Nc is exceeded, a compressorstop will occur, running counter to load following. Thus, the roomtemperature is raised back by increasing the frequency with the starconnection kept as it is and the connection is switched by stopping thecompressor 3 at B3 after a definite period of time from B2. After theswitch to the delta connection, the compressor is started at B4, andheating operation is resumed at B5 after going through defrostingoperation. When the compressor is stopped for connection switching, ifoperation of the indoor-side fan 8 is continued as it is, the user feelschilly and unpleasant, which makes it necessary to stop operation of theindoor-side fan 8. For effective use of this downtime, after the switchto the delta connection, by shifting to defrosting operation first, thefrost produced on the outdoor-side heat exchanger 6 by the precedingoperation is first melted, and then a shift is made to heating operationwith the delta connection. This makes it possible to delay the timing ofnext defrosting operation, and connection switching operations areincluded in the existing number of shutdowns by defrosting, whicheliminates the need to increase the number of compressor stops in anentire heating period. This enables shifting to the delta connectionwhile maintaining comfort.

During a shift from the star connection to the delta connection, the airconditioning load acts in such a direction as to require high capacity,and thus it is desirable that the time until a restart is short. Here,during the shift from the star connection to the delta connection, thetime taken to eliminate differential pressure in the refrigerant circuit100 is reduced by switching the four-way valve 4 and thereby reversing arefrigerant circulation before and after defrosting operation. That is,by inserting defrosting operation during switching from the starconnection to the delta connection in heating operation, the waitingtime for equalization of refrigerant pressure to prevent compressorfailures is reduced. Consequently, downtime for connection switching canbe reduced while curbing increases in the number of times of defrostingin the entire heating period, and switching can be done whilemaintaining comfort.

Air conditioning load fluctuations are temporary, and even if highfrequency is required temporarily, if the air conditioning load goestoward stabilization when the compressor frequency falls below thefrequency threshold again between B2 and B3 or if it is determined,based on air conditioning load detection information, estimationinformation, or other information, that operation at a frequency lowerthan the frequency threshold Nc will continue, switching from the starconnection to the delta connection does not have to be done. Forexample, if the compressor frequency is lower than Nc between B2 and B3,switching from the star connection to the delta connection does not haveto be done.

FIG. 12 is a schematic diagram showing an example of changes, with time,of compressor frequency during heating if target value fluctuationsoccur. During heating operation with the star connection, the targettemperature rises at time C1, and the compressor frequency increasesaccordingly. Since operation is performed at frequencies above thefrequency threshold Nc at time C1 a, even though it is appropriate tooperate using the delta connection, an attempt to perform a connectionswitching operation by stopping the compressor just after changing theset temperature runs counter to the intent of the user who is trying toincrease the room temperature and will cause the user to feel odd. Thus,the room temperature is increased with the star connection kept as it isuntil time C2, switching to the delta connection is done between C2 andC3, defrosting operation is started at C3, and heating operation isresumed at C4. When the indoor temperature reaches the targettemperature and thermo-off occurs between C1 and C2, the operation ofswitching to the delta connection does not have to be done.

During heating operation with the star connection, when it is determinedthat defrosting is needed based on the temperature detected by thetemperature sensor 12, i.e., the temperature of the outdoor-side heatexchanger 6, and a shift to defrosting operation is made, switching tothe delta connection is done at the time of a compressor stop beforedefrosting operation is started. Generally, it is more comfortable tofinish defrosting operation in a short time and quickly return toheating operation, and thus the delta connection that can exhibit highdefrosting capacity is always selected during defrosting operation.

When a sudden load fluctuation makes it necessary to shift from the starconnection to the delta connection, priority is given to comfort, andconsequently efficiency is not necessarily optimal. However, when thestar connection is used, because the frequency at which too sudden loadfluctuations to follow will occur is low in view of the actual status ofuse, load fluctuations do not have a significant impact on annual powerconsumption under standard conditions. Also, to make it possible tofollow sudden load fluctuations using the star connection, the number ofmotor windings is selected in such a way as to enable operation at ratedfrequency even with the star connection.

Next, a control method in which the above-mentioned conditions forswitching from the star connection to the delta connection are reflectedwill be described based on FIGS. 13 to 15. In the present embodiment,defrosting operation and connection switching are synchronized with eachother.

FIG. 13 is a flowchart (Part 1) of connection switching control forswitching the stator windings of the electric motor from the starconnection to the delta connection, showing an example in whichoperating time and outdoor-side heat exchanger temperature are used asstart conditions of defrosting operation. The connection switchingcontrol is performed when the stator windings of the permanent magnetmotor 25 are star-connected. The controller 21 determines whether adefinite period of time has passed since the start of heating operationafter an end of defrosting (S31). When it is determined that a definiteperiod of time has passed, next the controller 21 determines whether thetemperature detected by the temperature sensor 12 is equal to or lowerthan the prescribed value (Tdef_on) (S32). When it is determined thatthe temperature detected by the temperature sensor 12 is equal to orlower than the prescribed value (Tdef_on), the controller 21 determinesthat a condition for starting defrosting operation is satisfied.However, in the present embodiment, instead of shifting to defrostingoperation immediately, first the controller 21 stops the compressor 3(S33). With the compressor 3 stopped, the controller 21 switches fromthe star connection to the delta connection by controlling theconnection switching device 24 (S34). Subsequently, the controller 21starts defrosting operation using the delta connection (S35). That is,the controller 21 forms a cooling circuit by switching the four-wayvalve 4 and melts frost by the reverse defrosting method that involvesrestarting the compressor 3 and circulating the refrigerant. Thedefrosting operation is continued until the temperature detected by thetemperature sensor 12 reaches or exceeds the prescribed value (Tdef_off)(S36). When the temperature detected by the temperature sensor 12reaches or exceeds the prescribed value (Tdef_off), the controller 21finishes the defrosting operation (S37). In finishing the defrostingoperation, first the controller 21 stops the compressor 3, returns tothe heating circuit by switching the four-way valve 4, and resumesheating operation by restarting the compressor 3 (S38). When theconditions are not satisfied in steps S31 and S32, i.e., when thedeterminations are NO, the processing is finished. Note that step 31(S31) above may be omitted.

FIG. 14 is a flowchart (Part 2) of connection switching control forswitching the stator windings of the electric motor from the starconnection to the delta connection, showing an example in whichcompressor frequency is used as a start condition of defrostingoperation. The process of the flow chart is performed periodically. Thecontroller 21 determines whether the compressor frequency is equal to orhigher than the frequency threshold (S41). When it is determined thatthe compressor frequency is equal to or higher than the frequencythreshold, the built-in timer 21 a is incremented (S42). When thecondition is not satisfied in step 41 (S41), the timer 21 a is reset(S44). The controller 21 determines whether the value of the timer 21 ais equal to or higher than a default value (S43). When the value of thetimer 21 a is equal to or higher than the default value, the controller21 goes to step 33 (S33) of FIG. 13 by determining that the conditionfor the defrosting has been satisfied and carries out step 33 (S33) andsubsequent processes of FIG. 13.

FIG. 15 is a flowchart (Part 3) of connection switching control forswitching the stator windings of the electric motor from the starconnection to the delta connection, showing an example in which thetemperature of the compressor 3 is used as a start condition ofdefrosting operation. The controller 21 determines whether thetemperature detected by the temperature sensor 10, i.e., the temperatureof the compressor 3, exceeds reference temperature (S47). When thereference temperature is exceeded, the controller 21 goes to step 33(S33) of FIG. 13 and carries out step 33 (S33) and subsequent processesof FIG. 13. Note that the reference temperature in step 47 (S47) aboveis set from the viewpoint of protecting the compressor 3. The abovedescription assumes heating operation, and in the case of coolingoperation, the cooling operation is continued after the connectionswitching process by omitting defrosting control.

Variations

An example focusing on compressor frequency has been described above inFIG. 14. However, because there is a correlation between the frequencyand operating current of the compressor 3, the operating current of thecompressor 3 may be used instead of the compressor frequency. FIG. 16,which corresponds to FIG. 11, is a schematic diagram showing an exampleof changes, with time, of the compressor operating current duringheating if load fluctuations occur. In this example, connection isswitched under conditions that a state in which the room temperature islower than the target temperature and the operating current of thecompressor 3 is higher than an operating current threshold Ic continuesfor a definite period of time.

FIG. 17 is a variation of FIG. 14 and is an example in which theoperating current of the compressor 3 is used as a start condition ofdefrosting operation. Step 41 (S41) of FIG. 14 is replaced with step 41a (S41 a). The controller 21 determines whether the operating current ofthe compressor 3 is equal to or higher than the operating currentthreshold Ic (S41 a). When it is determined that the operating currentof the compressor 3 is equal to or higher than the operating currentthreshold Ic, the built-in timer 21 a is incremented (S42). The otherprocesses are the same as the processes of FIG. 14. Connection switchingand defrosting are done under a condition that a state in which theoperating current of the compressor 3 is higher than the operatingcurrent threshold Ic continues for a definite period of time.

As described above, the present embodiment switches connection from thestar connection to the delta connection, offering the followingadvantageous effects.

In the present embodiment, to perform defrosting operation with thestator windings star-connected during heating operation, the connectionof the stator windings is switched from the star connection to the deltaconnection. That is, switching from the star connection to the deltaconnection is done in synchronization with defrosting operation. Thismakes it possible to switch connection without increasing the stopfrequency of the compressor 3 compared to conventional stop frequencyand thereby reduce unpleasantness caused by a compressor stop duringconnection switching. Besides, the shift from the star connection to thedelta connection provides high capacity in a high-load region. Also, theaccuracy of determination as to which connection is more appropriate foroperation is increased. This makes it possible to reduce unnecessaryconnection switching and maintain comfort. In this way, the presentembodiment combines energy efficiency in a low-load region and highcapacity in a high-load region without impairing comfort.

Also, in the present embodiment, when a state in which the roomtemperature is lower than the target temperature continues for a firstreference time, the connection of the stator windings is switched fromthe star connection to the delta connection. That is, connectionswitching from the star connection to the delta connection is done inconjunction with defrosting operation. This makes it possible to switchconnection without increasing the stop frequency of the compressor 3compared to conventional stop frequency and thereby reduceunpleasantness caused by a compressor stop during connection switching.Besides, the shift from the star connection to the delta connectionprovides high capacity in a high-load region. Also, the accuracy ofdetermination as to which connection is more appropriate for operationis increased. This makes it possible to reduce unnecessary connectionswitching and maintain comfort. In this way, the present embodimentcombines energy efficiency in a low-load region and high capacity in ahigh-load region without impairing comfort.

Also, in the present embodiment, during operation with the statorwindings star-connected, when the temperature of the outdoor-side heatexchanger 6 is equal to or lower than the first reference temperature,the connection of the stator windings is switched from the starconnection to the delta connection using the connection switching device24. This further increases accuracy of determination on switching to thedelta connection.

Also, in the present embodiment, during operation with the statorwindings star-connected, when a state in which the compressor frequencyis equal to or higher than the frequency threshold continues for thesecond reference time, the connection of the stator windings is switchedfrom the star connection to the delta connection. This further increasesthe accuracy of determination on switching to the delta connection.

Also, in the present embodiment, during operation with the statorwindings star-connected, when a state in which the operating current ofthe compressor 3 is equal to or higher than the operating currentthreshold continues for the second reference time, the connection of thestator windings is switched from the star connection to the deltaconnection. This further increases the accuracy of determination onswitching to the delta connection.

Also, in the present embodiment, during operation with the statorwindings star-connected, when the temperature of the compressor 3exceeds the reference temperature, the connection of the stator windingsis switched from the star connection to the delta connection. Theoperation in this case may be either heating or cooling, and from theviewpoint of protection of the compressor 3, the delta connection isused for the stator windings. However, during heating operation, a shiftto defrosting operation is made after connection switching.

Also, in the present embodiment, the defrosting operation is performedafter the connection of the stator windings is switched from the starconnection to the delta connection. That is, the switching from the starconnection to the delta connection is done in synchronization withdefrosting operation. This makes it possible to switch connectionwithout increasing the stop frequency of the compressor 3 compared toconventional stop frequency and thereby reduce unpleasantness caused bya compressor stop during connection switching.

Also, in the present embodiment, by stopping the compressor 3, theconnection switching device 24 is caused to switch connection, with thecompressor 3 stopped. Consequently, even if there is variation inoperating characteristics of the C contact relays 24 a to 24 c making upthe connection switching device 24, safety of products is ensuredwithout inconvenience.

Reference Signs List 1 indoor unit 2 outdoor unit 3 compressor 4four-way valve 5 indoor-side heat exchanger 6 outdoor side heatexchanger 7 expansion valve 8 indoor-side fan 9 outdoor-side fan 10 to12 temperature sensor 16a, 16b glass terminal 21 controller 21a timer 22storage device 23 drive circuit 24 connection switching device 24a to24c C contact relay 25 permanent magnet motor 100 refrigerant circuit

The invention claime is:
 1. An air-conditioning apparatus comprising: acompressor incorporating an electric motor; a temperature sensorconfigured to detect indoor temperature; a driver configured to drivethe electric motor; a connection switch configured to switch connectionof stator windings of the electric motor between a first connectionstate and a second connection state higher in line-to-line voltage thanthe first connection state; a controller configured to enter thermo-offwhen the indoor temperature reaches one of a target temperature or acorrection temperature that is set based on the target temperature, andconfigured to cause the connection switch to switch connection, thethermo-off being entered by stopping the compressor via the driver; anda temperature sensor configured to detect temperature of the compressor,wherein the controller causes the connection switch to switch theconnection of the stator windings during the thermo-off, and with theelectric motor operating in the first connection state, the controllerdoes not control the connection switching to switch from the firstconnection state to the second connection state when the temperature ofthe compressor is equal to or higher than a reference temperature, evenif a condition for switching from the first connection state to thesecond connection state is satisfied.
 2. The air-conditioning apparatusof claim 1, wherein a thermo-off count is incremented when thecontroller enters the thermo-off and an operating frequency of thecompressor immediately prior to the thermo-off is equal to or lower thana frequency threshold, and when the thermo-off count reaches a referencecount, the controller causes the connection switch to switch theconnection from the first connection state to the second connectionstate.
 3. The air-conditioning apparatus of claim 1, wherein when aduration of the thermo-off exceeds a reference time, the controllercauses the connection switch to switch the connection from the firstconnection state to the second connection state.
 4. The air-conditioningapparatus of claim 1, wherein when a reference time is exceeded during aperiod from a first count of thermo-off until a reference count isreached, the controller resets a thermo-off count.
 5. Theair-conditioning apparatus of claim 1, wherein when the controllerperforms a defrosting operation to remove frost from an outdoor-sideheat exchanger, the controller resets a thermo-off count.
 6. Theair-conditioning apparatus of claim 1, wherein the controller does notcontrol the connection switching before elapse of a reference time froma start of operation of the air-conditioning apparatus.
 7. Anair-conditioning apparatus comprising: a compressor incorporating anelectric motor; a temperature sensor configured to detect indoortemperature; a driver configured to drive the electric motor; aconnection switch configured to switch connection of stator windings ofthe electric motor between a first connection state and a secondconnection state higher in line-to-line voltage than the firstconnection state; a controller configured to enter thermo-off when theindoor temperature reaches one of a target temperature or a correctiontemperature that is set based on the target temperature, and configuredto cause the connection switch to switch connection, the thermo-offbeing entered by stopping the compressor via the driver; and atemperature sensor configured to detect a temperature of the compressor,wherein the controller causes the connection switch to switch theconnection of the stator windings during the thermo-off, and duringoperation with the stator windings being in the second connection state,when temperature of the compressor exceeds a reference temperature, thecontroller causes the connection switch to switch the connection of thestator windings from the second connection state to the first connectionstate.
 8. The air-conditioning apparatus of claim 1, further comprisinga storage device wherein the storage device prestores a reference countand stores a thermo-off count.
 9. The air-conditioning apparatus ofclaim 1, wherein in the first connection state, the stator windings aredelta-connected, and in the second connection state, the stator windingsare star-connected.
 10. The air-conditioning apparatus of claim 1,wherein in the first connection state, the stator windings are connectedin parallel on a phase by phase basis, and in the second connectionstate, the stator windings are connected in series on a phase by phasebasis.
 11. The air-conditioning apparatus of claim 1, wherein when athermo-off count reaches a reference count, the controller causes theconnection switch to switch the connection of the stator windings fromthe first connection state to the second connection state.